In the dry-process electrophotography, electrophotographic functional component parts such as a blade and a feed roller apply pressure or the like so that a toner is triboelectrically charged by the frictional force produced between the toner and the surfaces of such electrophotographic functional component parts to make the toner carried on other electrophotographic functional component part such as a developing roller. Accordingly, as a surface layer of an electrophotographic functional component part, it is necessary to select a negatively chargeable material when the toner should positively be charged, and a positively chargeable material when the toner should negatively be charged. As the former material, it is proposed to use a fluorine resin or the like, and as the latter material a nylon resin, a urethane resin or the like.
Meanwhile, in the electrophotographic functional component part, it is preferable to use a soft base material layer composed chiefly of a rubbery material, in order to, e.g., better secure the nip width and lessen the stress to toner. However, where such a chief soft base material layer is used, the frictional force alone which is produced between the toner and the electrophotographic functional component part surface may provide a small charge quantity, and it may be difficult to obtain good images by merely using the above fluorine resin, nylon resin or urethane resin proposed conventionally.
In order to resolve such a problem and taking account of intentionally improving charge characteristics of surface layers of electrophotographic functional component parts, a means is employed in which a charge control agent is added to electrophotographic functional component parts coming into contact with the toner. Recently, a method is proposed in which a charge control resin soluble in an organic solvent and capable of being coated on a developing roller or a developing sleeve is used as the charge control agent, and a method is reported in which it is used in a developing sleeve (e.g., Japanese Patent Applications Laid-open No. 2000-242033, No. 2002-244426 and No. 2003-005507). In all of these, a copolymer of an amino-group-containing acrylate or methacrylate monomer and methyl methacrylate is used as the charge control resin.
The present inventors have invented that the methyl methacrylate of the charge control resin disclosed in, e.g., the above Japanese Patent Applications Laid-open No. 2000-242033, No. 2002-244426 and No. 2003-005507 is replaced with an acrylate or methacrylate monomer having an alkyl group having 4 or more carbon atoms and this is copolymerized with an amino-group-containing monomer to obtain, when added to a base material resin, a charge control resin which can provide a larger charge quantity (Japanese Patent Applications Laid-open No. 2005-31656 and No. 2005-31657). It is considered that, when added to the base material resin, the alkyl group having 4 or more carbon atoms which is contained in the acrylate or methacrylate monomer prompts the surface orientation of amino groups contained in the amino-group-containing monomer, to promise a larger charge quantity.
The addition of the above charge control resin to the surface layer of the electrophotographic functional component part brings an improvement in image characteristics which is seen in virtue of the charge quantity made larger. It, however, has been found on the other hand that a difficulty on image characteristics which is due to the addition of such a charge control resin is seen depending on conditions of image evaluation.